The present invention relates generally to methods of improving composite surface bonding characteristics, and more particularly to methods of improving composite surface bonding characteristics through the application of a fiber embedded peel-ply sheets.
In modern aircraft, the surface skins commonly take the form of composite laminates. Such composite materials comprise fibers embedded or impregnated in a resin matrix. The fibers are typically formed from carbon or glass. Often, it is necessary to attach to aircraft skins various components such as internal structural supports (i.e., stiffeners). In the case of a damaged aircraft skin section, repairs can take the form of attachment of a repair patch to the skin. In these and other instances adhesive bonding is usually desirable. The strength of the adhesive bond to composite materials is a function of the surface characteristics.
Commonly, the surface of composite materials has a glossy surface resin layer. This relatively smooth, mirror-like layer of resin is formed from excess resin which tends to flow and pool at the surface of the resin matrix once the embedded fibers are saturated during the curing process. Because the adhesive or bonding strength is a function of surface area, such a glossy surface exhibits a relatively low bonding strength.
As a result, there has been industry drive to develop low cost methods to increase the surface area of composite materials. One such development in the art is the use of peel-ply sheets during the manufacture of resin composite structures. A conventional peel-ply sheet consists of a sheet of woven material. The woven material is placed on the uncured resin surface of composite structures. The resin is allowed to saturate the peel-ply material. Subsequently, the resin composite with the applied peel-ply material is allowed to cure. Once the resin hardens, the peel-ply material is removed or peeled away from the resin composite. As a result, the surface of the resin composite is imprinted with texture characteristics similar to the peel-ply material. Accordingly, the surface area of the resin composite is increased. The textured surface of the resin, though being irregular, however, remains relatively smooth on a micro-level.
As such, whether or not a peel-ply procedure is utilized, further surface preparation techniques have been developed to increase the surface area of cured resin composites. A common technique is to perform manual sanding of the surface of the resin composite. As a result of sanding, the surface becomes roughened and the surface area is accordingly increased. Such an additional process, however, is labor intensive and therefore is relatively expensive.
Sand blasting or similar pressurized surface abrasion techniques are a more drastic approach to increasing resin composite surface area. While such procedures are effective in rapidly roughening the resin composite surface, controlling the degree of abrasion is difficult. Over processing the surface can result in damage to the resin composite structure. This occurs at areas where the surface resin is entirely removed and underlying composite fibers become exposed. Exposure of composite fibers results in the weakening of the structural integrity of the composite and is therefore highly undesirable. In addition, over processing can result in a polishing of the subject surface with a corresponding decrease of the surface area.
Moreover, with regard to surfacing processes, such as sanding, such processes only affect localized portions of the resin surface where a peel-ply application is utilized. Equating the textured surface to a series of mountains and valleys, sanding only effects the uppermost portions of the mountains. Thus, the surface at the lower portions of the mountains and the valleys would be unaffected by the sanding procedure. Thus, these unaffected regions would retain their glossy nature and would continue to exhibit relatively poor bonding characteristics.
It is therefore evident that there exists a need in the art for a low cost method of increasing the surface area of resin composites to improve surface bonding characteristics.
In accordance with the present invention, there is provided a method of increasing the surface bond strength characteristics of a resin composite material. The method begins with providing a peel-ply sheet which defines opposed planar faces. A multitude of elongate bond fibers are embedded into the peel-ply sheet in a manner in which a majority of the bond fibers do not extend in co-planar relation to the planar faces of the peel-ply sheet. A resin composite material is provided. The peel-ply sheet is applied to the resin composite material such that the bond fibers are partially embedded therein. The resin composite material is cured. The peel-ply sheet is removed from the cured resin composite material such that the bond fibers remain implanted therein.
The planar faces of the peel-ply sheet may be defined by an X-Y plane. The bond fibers are initially embedded in the peel-ply sheet such that the fibers are oriented to have a Z-axis component with resect to the planar faces of the peel-ply sheet. When the bond fiber embedded peel-ply sheet is applied to the uncured resin composite material, the bond fibers are partially embedded in and partially extending from the resin composite material. As a result, the cured resin composite material has an increased effective surface area which includes the surface areas of the exposed portions of the implanted bond fibers. Thus, a significant increase in adhesive bonding strength of the resin composite material is realized.
It is contemplated that the peel-ply material may be in the form of a fabric, which is woven or matted. A polymer material is preferably used. The material selection of the peel-ply sheet material is subject to being able to withstand the curing temperatures of the resin composite material to which it is applied. Thus, it is desirable that the peel-ply sheet material be heat resistant. In addition, the peel-ply sheet is preferably coated with a release agent to facilitate removal and separation of the peel-ply sheet from the bond fibers and the cured resin composite material. With respect to the bond fibers, a carbon material or graphite material may be used.
In the preferred embodiment of the present invention, the bond fibers are embedded into the peel-ply sheet through the use of a flocking process. The flocking process may be mechanical or electro-static in nature. The flocking process includes shooting the bond fibers at the peel-ply sheet with sufficient force to embed the bond fibers into the peel-ply sheet. The bond fibers are shot from a bond fiber source through a screen, with a portion of the bond fibers passing through the screen and becoming embedded in the peel-ply sheet. The bond fibers after having passed through the screen are imbedded in the peel-ply sheet in a series of bushels. The bond fibers of each bushel extending in multiple directions relative to the planar faces of the peel-ply sheet.
In addition, the present invention includes the fiber embedded peel-ply sheets constructed in accordance with the above described methods.
In another embodiment of the present invention, there is provided a method of increasing the surface bond strength characteristics of a resin composite material. The method begins with providing a peel-ply sheet formed of intermeshed transfer fibers and bond fibers. Preferably the transfer fibers to have a relatively higher tensile strength than the bond fibers and transfer fibers and bond fibers are woven together. A resin composite material is provided. The peel-ply sheet is applied to the resin composite material such that the bond fibers are partially embedded therein. The resin composite material is cured.
The peel-ply sheet is removed from the cured resin composite material such that the transfer fibers are separated from the resin composite material and the bond fibers are fractured so as to form bond fiber fragments. As a result the bond fiber fragments are partially embedded in and protrude from the cured resin composite material.
It is contemplated that the transfer fibers may be a polymer material and is heat resistant to withstand the curing temperatures of the resin composite material to which it is applied. In addition, the transfer fibers are preferably coated with a release agent to facilitate removal and separation from the bond fibers and the cured resin composite material. With respect to the bond fibers, a carbon material or graphite material may be used
In addition, the present invention includes the resin composite materials which have been implanted with the bond fibers or bond fragments in accordance with the above described methods.
As such, based on the foregoing, the present invention mitigates the inefficiencies and limitations associated with prior art methods to increase the surface area of resin composite materials and structures.
In comparison to using conventional peel-ply sheets, the bond fibers utilized in the present invention significantly increase the surface area. A significant increase in adhesive bonding strength is thus realized. Significantly, the present invention avoids the potential for damaging the subject composite material as is the case with over-sanding during abrasive surface processes.
In addition, where separate resin composite structures are formed using peel sheets of the present invention, it is contemplated that the impregnated bond fibers of the respective composites structures would interlock during bonding of the structures. Such interlocking further increases the adhesive bonding strength.
It is contemplated that the peel-ply sheets of the present invention would be especially efficient in the resin composite fabrication shop environment. A composite fabrication worker need only have a roll of the peel-ply sheets available for use. Flocking equipment or other similar devices for implanting the bonding fibers need not be purchased by the composite manufacturer. The composite manufacturer would simply purchase a roll or rolls of the bond fiber impregnated peel-ply sheets, just as would be customarily done with any conventional peel-ply sheets. Thus, no specialized tooling need be purchased by the manufacturer of the composite material in order to practice the present invention.
Advantageously, in terms of labor costs, in order to practice the present invention, the composite fabrication worker needs only to spend a comparable amount of time applying and removing the bond fiber impregnated peel-ply sheets as would be needed with regard to use of conventional peel-ply sheets.
Accordingly, the present invention represents a significant advance in the art.